120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
It is: 9C7 = 36
9^9 = 387420489 different digit combinations.
If there are no restrictions on duplicated numbers or other patterns of numbers then there are 10 ways of selecting the first digit and also 10 ways of selecting the second digit. The number of combinations is therefore 10 x 10 = 100.
If each can be used again, it is 7x7x7x7 or 2401 combinations. If not it is 7x6x5x4 or 840 combinations
Through the magic of perms and coms the answer is 729
Each digit can appear in each of the 4 positions. There are 9 digits, therefore there are 9⁴ = 6561 such combinations.
10,000 Because it begins with 0000, 0001, 0002... and goes to 9999.